1. Field of the Invention
The present invention relates to an IC module in which an IC chip and tuning capacitor are integrated, and to a wireless information-storage medium or a wireless information-transmitting/receiving apparatus, such as a contactless IC card, including the IC module.
2. Description of the Related Art
A wireless information-storage medium of the type mentioned above (hereinafter referred to as an IC card) includes a loop antenna, an IC chip connected to terminals of the loop antenna, and tuning capacitors formed on a principal surface of a plastic card substrate. The wireless information-storage medium is used, for example, for checking transportation tickets or management of room entry and exit.
When the contactless IC card is placed in proximity to a loop antenna of a reader/writer (i.e., a device for reading and writing) that is provided separately, an induced voltage is generated between the terminals of the loop antenna of the IC card by electromagnetic induction. The IC chip receives a power-supply voltage obtained by stabilizing the induced voltage, and demodulates a modulated wave transmitted from the reader/writer, thereby receiving data transmitted from the reader/writer. When data stored in a memory of the IC chip is to be transmitted to the reader/writer, a load in the IC chip is varied according to the data to change the terminal impedance of the loop antenna, and the reader/writer demodulates a modulated wave transmitted from the IC card to detect the variation of the load, thereby receiving the data from the IC card.
When the contactless IC card is manufactured, capacitors for tuning the signal frequency f of an antenna coil are also formed on the substrate. Since the signal frequency f of the antenna coil can be expressed as f=1/(2π√(LC)), where C denotes the capacitance of the capacitors and L denotes the inductance of the antenna coil, the signal frequency of the antenna coil is tuned by adjusting the capacitance of the capacitors.
More specifically, with a polyimide film or a polyethylene naphthalate film forming the substrate as an insulator, electrodes of copper or aluminum are formed on both sides of the film, whereby film capacitors are formed on the substrate.
The film capacitors constructed as described above, however, suffer low precision of capacitance because of non-uniformity in the thickness of polyimide films or polyethylene naphthalate films manufactured. Thus, a final product actually requires a delicate adjustment of capacitance by trimming.
Furthermore, since variation of dielectric loss tangent (tanθ) in relation to temperature change is large in capacitors that are formed using a polyimide film or a polyethylene naphthalate film, it is inhibited to satisfy desired temperature characteristics of the tuning capacitors.
As for multilayer chip capacitors, although manufacturing precision of capacitance is high and temperature characteristics are favorable, a thickness on the order of 300 μm inhibits their use in very thin devices such as IC cards.